During recent years, there has been an increasing interest in the development of sophisticated devices capable of sensing the 6 degrees of freedom motion of a single or multiple axes rigid body in 3 dimensional space. The devices which have been developed to sense these motion components have, in the past, been rather bulky, high power, and expensive. For instance, in order to sense angular and linear motion, both gyroscopes and accelerometers have typically been used in tandem to provide the motion information necessary for most applications. However, these devices have been disfavored because of their bulk, high power requirements, and high cost.
There have been improvements in recent devices which require less power, are more precise, and slightly less bulky than prior devices. These more modern motion sensing devices, however, are still rather bulky and as a result, are often difficult to integrate into the device which is to be measured. This is particularly problematic when there is a desire to measure the motion of a small, lightweight device, such as a piece of sports equipment where the addition of bulky hardware would change the nature of the movement or use of the device.
One of the earliest attempts at a miniaturized motion sensing device is disclosed in U.S. Pat. No. 4,718,276 which issued in January of 1988 to Laughlin for an invention entitled “Angular motion sensor.” The '276 patent discloses a solid electrode in the core of an angular motion sensor has a body of conductive fluid confined therein within an annular flux gap between axially spaced magnets. An arrangement of slots in the walls of the electrode modifies the current, induced in the fluid by inertial displacement, into a circumferential component, which is inductively coupled to an output winding from which an output signal is obtained.
Attempts to decrease the physical size of motion sensing components continued as presented in U.S. Pat. No. 5,831,162 which issued in November of 1998 to Sparks for an invention entitled “Silicon micromachined motion sensor and method of making” discloses a method for making and vacuum packaging a silicon micromachined motion sensor, such as a gyroscope, at the chip level. The method involves micromachining a trench-isolated sensing element in a sensing chip, and then attaching a circuit chip to enclose the sensing element. Solder bumps serve to attach the circuit chip to the sensing chip, form a hermetic seal to enable vacuum-packaging of the sensor, and electrically interconnect the sensing chip with the circuit chip. Conductive runners formed on the enclosed surface of the circuit chip serve to electrically interconnect the sensing element with its associated sensing structures.
The recent development of lightweight angular and linear motion sensors involving MEMS components has led to innovations such as that disclosed in U.S. Pat. No. 6,504,385 which issued in January of 2003 to Hartwell for an invention entitled “Three-axis motion sensor.” The '385 patent discloses a microelectromechanical system (MEMS) motion sensor for detecting movement in three dimensions of a semiconductor wafer structure.
The MEMS device has top, middle, and bottom layers, with a movable portion, or “mover,” attached to the middle layer by a flexure that allows the mover to move in three dimensions relative to the layers. The mover has mover electrodes that create a capacitance with counter electrodes positioned on an adjacent layer. The capacitance changes as the mover moves. A capacitance detector receives signals from the electrodes and detects movement of the mover based on the change in capacitances. The MEMS device processes the detected capacitances to determine the nature of the movement of the mover. The mover and counter electrodes comprise x-y electrodes for detecting movement in an x-y plane parallel to the middle layer and z electrodes for detecting movement in a direction orthogonal to the x-y plane.
While the device of the '385 patent is capable of providing measured signals corresponding to three axes of freedom, it nevertheless does not provide rate information for overall motion of the device.
Continued development of MEMS sensors includes a sensor as presented in U.S. Pat. No. 6,513,380 which issued in February of 2003 to Reeds for an invention entitled “MEMS sensor with single central anchor and motion-limiting connection geometry.”
The '380 patent discloses a MEMS sensor including a sense element and a single anchor that supports the sense element arranged in a central hub-like fashion that reduces the effects of thermal stress. Usually, two or more anchors are required to suitably constrain the sense element's motion. The anchor disclosed in the '380 patent, however, supports the sense element with connection elements having a connection geometry that substantially limits the motion of the sense element to a single-degree-of-freedom.
The incorporation of MEMS sensors into motion capture devices provides for a much lighter solution than typical motion sensors. However, the device of the '380 patent fails to account for the directional signals typically provided by a gyroscope in other sensors, and thus, is not useful as a complete motion sensor component. Further, the method of attachment of the various MEMS components does not provide a robust sensor capable of incorporation into items being measured.
An alternative solution to motion sensing is presented in U.S. Pat. No. 6,552,531 which issued in April of 2003 to Fey for an invention entitled “Method and circuit for processing signals for a motion sensor.” The '531 patent discloses a method and a circuit arrangement for processing signals for an active motion sensor which generates at least one first sequence of input pulses that contain motion information. By at least one integrating filter circuit, each input pulse of a pulse train is integrated, and an associated output pulse is generated during a period in which the integrated signal is in excess of a predeterminable second threshold value after a predeterminable first threshold value has been exceeded so that the output pulse has a time delay with respect to the input pulse. As a result, noises of a duration which is shorter than the delay time are effectively suppressed.
The methods for minimizing noise and improving the quality of the motion captured signal taught in the '531 patent make this device impractical for motion capture applications involving higher rates of change. This is particularly so given the delays which are necessarily implemented into the sensing circuitry to improve its noise tolerance, and thus make this device unresponsive for providing motion information for rapidly moving items.
A more recent solution that has been proposed for measuring motion is presented in U.S. Pat. No. 6,584,846 which issued in July of 2003 to Wesselak for an invention entitled “Magnetic motion sensor.” The '846 patent discloses a magnetic motion sensor, having a mobile magnet that generates an essentially homogeneous magnetic field with a magnetic-field direction, and having a coupling element which is stationary within the magnetic field, and wherein a motion-dependent physical quantity is induced in the coupling element when the magnet moves perpendicular to the magnetic-field direction, and the induced quantity is measured and output by a sampling element.
While the device disclosed in the '846 patent may generate a motion-based signal that is measurable, it is woefully susceptible to external magnetic fields. As a result, this device is not particularly useful in applications where the magnetic field may vary over time, or may vary between uses.
Most recently, United States Patent Application No. 20040211258 was published in October of 2004 by Geen for an invention entitled “Six degree-of-freedom micro-machined multi-sensor.” The '258 application discloses a six degree-of-freedom micro-machined multi-sensor that provides 3-axes of acceleration sensing, and 3-axes of angular rate sensing, in a single multi-sensor device. The six degree-of-freedom multi-sensor device includes a first multi-sensor substructure providing 2-axes of acceleration sensing and 1-axis of angular rate sensing, and a second multi-sensor substructure providing a third axis of acceleration sensing, and second and third axes of angular rate sensing. The first and second multi-sensor substructures are implemented on respective substrates within the six degree-of-freedom multi-sensor device.
In light of the above, there is a need to provide a motion sensing apparatus and system that is capable of sensing the spatial 6 degrees of freedom and which is relatively small, lightweight, low power suitable for portable applications, and relatively cost competitive.